Decompiling and Modifying a .NET Application using dnSpy

General
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Decompiling and Modifying a .NET Application using dnSpy

Introduction

In this article, we will explore the process of decompiling, modifying, and recompiling a .NET application using the dnSpy tool. We will start by creating a simple “Hello World” console application, and then use dnSpy to decompile the code, make changes, and recompile the application.

Tools and Software

For this article, we will be using the dnSpy tool, which can be downloaded from the official GitHub repository: https://github.com/0xd4d/dnSpy/releases/download/v4.0.0/dnSpy.zip

Creating a “Hello World” Console Application

To start, we will create a simple console application that displays the message “Hello World!” to the console. The code for this application is as follows:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace ChangeString
{
    class Program
    {
        static void Main(string[] args)
        {
            string str = "Hello World!";
            Console.WriteLine(str);
            Console.ReadKey();
        }
    }
}

Compiling and Running the Application

To compile and run the application, we can use the dnSpy.exe or dnSpy-x86.exe file, depending on whether we are using a 64-bit or 32-bit version of the software. Note that the corresponding debugging version should be used.

Once the application is compiled and run, we can see that the internal procedure variable str is set to “Hello World!”. However, we need to use dnSpy to modify this variable and debug the application.

Decompiling the Application with dnSpy

To decompile the application with dnSpy, we can follow these steps:

① Decompile the Application: Using the tool generated by dnSpy, decompile the ChangeString.exe application. This will create a decompiled version of the code that we can modify.

② Edit the Main Function: In the decompiled code, right-click on the Main function and select “Edit Method (C#)” from the context menu. This will open the Main function in the dnSpy editor.

③ Modify the str Variable: In the Main function, modify the str variable to “Just For Test”. This will change the value of the variable in the decompiled code.

④ Compile the Modified Code: Click on the “Compile” button to recompile the modified code.

⑤ Save and Run the Application: Click on the “Save All” button to save the modified code, and then click on the “Start” button to run the application. The application should now display the modified message “Just For Test” to the console.

Important Notes

  • When compiling the modified code, please note the following:
    • If there are other DLL modules, you must include them.
    • If the decompiled code has unrecognizable characters, you may need to manually modify the code before compiling it.
  • This program is not only effective for EXE files, but also for DLL files. Please test on your own and share your results.

Conclusion

In this article, we have explored the process of decompiling, modifying, and recompiling a .NET application using the dnSpy tool. We have created a simple “Hello World” console application, decompiled it with dnSpy, modified the code, and recompiled the application. This process demonstrates the power and flexibility of the dnSpy tool, and can be useful for a variety of purposes, including debugging, reverse engineering, and code modification.